Inter-base station synchronization system, synchronization control device, and base station

ABSTRACT

A base station synchronization system, a synchronization controller, and a base station are provided that are capable of establishing precise synchronization among a plurality of base stations. In a base station synchronization system including a plurality of base stations and a base station concentrator, the base station concentrator includes a control information generator for generating synchronization control information, and each base station includes a VCO oscillating at a frequency corresponding to an input control voltage, and a corrector for correcting the input control voltage to the VCO according to the synchronization control information. This configuration makes it possible to establish synchronization among the master clocks of the plurality of base stations so as to suppress phase differences among the base stations and precisely synchronize the base stations with each other.

TECHNICAL FIELD

The present invention relates to a base station that performs radiocommunication with mobile communications terminals, a synchronizationcontroller that provides control to establish synchronization among aplurality of base stations, and a base station synchronization systemincluding them.

BACKGROUND ART

Conventional mobile communications scheme like the W-CDMA (wideband-CodeDivision Multiple Access) defined by 3GPP (3rd Generation PartnershipProject) only standardize implementations in which base stations (calledNodeB in 3GPP) are directly connected to a base station controller(called RNC (radio Network Controller) in 3GPP)(See Non-patent Document1).

Also, in general, considering the configuration of the base stationcontroller, the number of base stations already used in service changesoftware, or change hardware by providing additional memory, forexample.

In contrast to the 3GPP, in connections between mobile communicationsnetworks such as PHS (personal Handyphone System) and public networks,there are techniques that use base station concentrators as higher-leveldevices that supervise the base stations. When such a base stationconcentrator is connected to a plurality of base stations to makeconnections with the base stations and public network, the base stationconcentrator supplies a frame synchronization signal to the basestations so that frame synchronization can be established among theplurality of base stations on the basis of the frame synchronizationsignal from the base station concentrator. (For example, see PatentDocument 1.)

-   Non-patent Document 1: 3GPP TS (Technical Specification) 25.401    V3.10.0, 2002-06, retrieved from the Internet [retrieved on    2005-01-24]: <URL:    http://www.3gpp.org/ftp/Specs/latest/R1999/25_series/25401-3a0.zip>.-   Patent Document 1: Japanese Patent Application Laid-Open No.    2002-094441-   Patent Document 2: Japanese Patent Application Laid-Open No.    08-237731 (1996)

DISCLOSURE OF THE INVENTION Problems to be Solved by the Invention

For example, with a base station controller standardized by 3GPP, thenumber of connected base stations cannot be increased even when basestations with smaller user accommodating capacities are connected inplace of base stations with larger user accommodating capacities, andthen the total number of accommodated users is reduced when such a basestation controller is used. Then, the number of users accommodated inthe system is reduced when base stations with smaller user accommodatingcapacities are provided in shadow areas where radio communication isdifficult.

In order to solve this problem, a technique is possible in which a basestation concentrator for supervising a plurality of base stations isused to make the base station controller recognize the plurality of basestations as a single base station. In such a technique, it is essentialto match the timing among the base stations connected to the basestation concentrator so that, for example, soft handover can beimplemented. However, in the conventional configuration in which thebase station concentrator supplies frame synchronization signal to thebase stations, the free-running counter values of the base stations maypresent discontinuous values due to deviations in phase of the masterclocks. Here, the free-running counter values are values outputted fromthe free-running counters provided in the base stations to supplycounter values to individual functional parts within the base stations.

In particular, in CDMA, the number of operating phenomena increases andthe amount of verification works in base stations increases depending onthe time positions of lost counter values and the amounts ofincrease/decrease of counter values. More specifically, for example,evaluations have to be made to see whether the circuit is set such thatthe free-running counter in a lower-level first base station connectedto the base station concentrator loads the initial value of spreadingcode of the modulator and demodulator, or whether the timing margin issufficient when a process that waits for the free-running counter of thefirst base station is shorter than expected.

Especially, in the case of CDMA base stations in which a plurality ofusers have to perform parallel operations according to various timingsby the FDD (Frequency Division Duplex), the number of such verificationworks exponentially increases ((all cases requiring timingverifications)^(the number of users)). Also, in Compressed-Mode of 3GPPor in process like broadcast channels that operate in extremely longcycles, just an instantaneous timing loss might render the entire systemoperations abnormal for a long time even if the unlikely event of atiming abnormality, because normalization operations are usually kept onstandby until the next cycle.

Also, when the wire transmission line between the base stationconcentrator and base stations is an ATM (Asynchronous Transfer Mode) orIP (Internet Protocol) circuit other than a dedicated line, it isimpossible to directly transmit clocks at hubs, routers and the like,and then circuit synchronization cannot be established in the basestations. Accordingly, propagation delays and drifts between the basestation concentrator and a base station will be considerably differentfrom propagation delays and drifts between the base station concentratorand another base station.

Under these circumstances, in conventional systems, radio frame timingdifferences among a plurality of base stations become larger and largeras time passes, no matter how precise the clock source may be, and thendata sent from the base station controller will arrive at a base stationtoo early and another base station too late, making normal communicationimpossible.

An object of the present invention is to provide a base stationsynchronization system, a synchronization controller, and a base stationwhich are capable of establishing precise synchronization among aplurality of base stations.

Means for Solving the Problems

According to the present invention, a base station synchronizationsystem includes a plurality of base stations and a synchronizationcontroller that provides control to establish synchronization among theplurality of base stations,

wherein said synchronization controller includes a control informationgenerator that generates synchronization control information forsynchronizing master clocks of said plurality of base stations with eachother, and

each said base station includes a master clock generator that oscillatesat a frequency corresponding to an input control voltage, and a controlvoltage corrector that corrects the input control voltage to said masterclock generator in accordance with the synchronization controlinformation generated by said synchronization controller.

The present invention also provides a synchronization controller thatprovides control to establish synchronization among a plurality of basestations,

and the synchronization controller includes a control informationgenerator that generates control information for synchronizing masterclocks of said plurality of base stations with each other.

The present invention also provides a base stations that synchronizeswith another base station,

-   -   and the base station includes a master clock generator that        oscillates at a frequency corresponding to an input control        voltage, and    -   a control voltage corrector that corrects the input control        voltage to said master clock generator in accordance with        synchronization control information that is provided from        outside.

EFFECTS OF THE INVENTION

According to the present invention, the master clocks of the pluralityof base stations are synchronized with each other, which makes itpossible to suppress phase differences among the base stations toprecisely synchronize the base stations with each other.

BEST MODE FOR CARRYING OUT THE INVENTION First Preferred Embodiment

FIG. 1 is a diagram illustrating a network configuration according to afirst preferred embodiment of the present invention. This networkincludes a base station controller 11, a plurality of base stations 13,and a base station concentrator 12 connected between them. The basestation controller 11 and the base stations 13 are W-CDMA equipmentdefined by 3GPP, for example, and the base stations 13 are devices thatperform radio communication with mobile communications terminals (notshown), and the base station controller 11 is a device of a higher levelthat is connected to the plurality of base stations 13. The base stationconcentrator 12 is connected with the plurality of base stations 13 of alower level, e.g., through an IP (Internet Protocol) network, for thepurpose of, e.g., making the higher-level base station controller 11recognize all base stations 13 connected thereto as a single basestation 13.

In this configuration, the base station concentrator 12 and theplurality of base stations 13 connected thereto constitute the basestation synchronization system of the invention.

The base station concentrator 12 includes a VCO (Voltage-ControlledOscillator) 20 and a control information generator 21. The VCO 20 isbuilt in the base station concentrator 12 to generate a master clock.The control information generator 21 generates synchronization controlinformation according to phase information about a master clockgenerated in each base station 13 (counter values based on the masterclock). Here, a comparison is made with phase information about themaster clock generated in the base station concentrator 12 (countervalues based on the master clock), a frequency deviation (a differencebetween counter values) is calculated as synchronization controlinformation corresponding to the result of the comparison, and a controlvoltage corresponding to the frequency deviation is generated as thesynchronization control information. The oscillator in the base stationconcentrator 12 may be, e.g., and OCXO (Oven-Controlled CrystalOscillator), in place of the VCO 20, when voltage control is notrequired.

Each base station 13 has a VCO 30. The VCO is built in the base station13 to generate the master clock.

Next, the operation illustrated in FIG. 1 will be described.

Each base station 13 informs the base station concentrator 12 offree-running counter values (time stamps), as the phase information,generated on the basis of the master clock oscillated by the VCO 30. InW-CDMA applications, the free-running counter values may be BFN (NodeBFrame Number). The base station concentrator 12 compares the phaseinformation about the master clock in the base station 13 (countervalues based on the master clock) and the phase information about themaster clock in the base station concentrator 12 (counter values basedon the master clock), and informs the base station 13 of the differenceas the synchronization control information. The, according to thesynchronization control information, the base station 13 corrects thecontrol voltage inputted to the VCO 30. The series of operations areperformed for all combinations of the base stations 13 (#1 to #n) andthe base station concentrator 12.

FIG. 2 is a diagram illustrating the detailed configurations of the basestation concentrator and a base station. The base station concentration12 further includes a free-running counter 22 and a phase comparator 23,in addition to the VCO 20 and the control information generator 21. Thefree-running counter 22 is a counter that is run by the VCO 20. Thephase comparator 23 includes subtracters 24, 25, a multiplier 26, and anintegrator 27, so as to compare counter values (phase information)provided by the free-running counter 22 in the base station concentrator12 and counter values (phase information) provided by a free-runningcounter 31 of the base station 13. The subtracter 24 subtracts thecounter value of the free-running counter 22 of the base stationconcentrator 12 from the counter value of the free-running counter 31 ofthe base station 13. The subtracter 25 subtracts the output value of theintegrator 27 from the difference between the counter values obtained bythe subtracter 24. The multiplier 26 multiplies together the outputvalue form the subtracter 25 and a time constant α (0<α<1). Theintegrator 27 continuously adds the output values from the subtracter 25multiplied by the time constant α. The subtracter 25, multiplier 26, andintegrator 27 constitute a loop circuit for averaging of the countervalues. The control information generator 21 stores a table indicating arelation between the frequency deviation and control voltage, and itgenerates a control voltage corresponding to the frequency deviation asthe result of comparison made by the phase comparator 23, and informsthe bas station of the control voltage as the synchronization controlinformation. The resultant value obtained by averaging the differencesbetween outputs of the free-running counter 22 of the base stationconcentrator 12 and outputs of the free-running counter 31 of the basestation 13 corresponds to an average of differences between the baseoscillation frequencies of the base station concentrator 12 and the basestation 13. The time for averaging the differences between countervalues (a period for averaging) can be changed according to the timeconstant α set for the multiplier 26. Experiential values obtained byexperiments are stored as the relation between the count valuedifferences and control voltage values.

Each base station 13 has the free-running counter 31, a DC power supply32, and a corrector 33, in addition to the VCO 30. The free-runningcounter 31 is run by the VCO 30 of the base station 13. The DI powersupply 32 supplies a control voltage to the VCO 30. The corrector 33corrects the control voltage supplied to the VCO 30 according to thesynchronization control information provided from the base stationconcentrator 12.

Next, the operation illustrated in FIG. 2 will be described.

Count values by the free-running counter 31 of the base station 13 arereported to the base station concentrator 12 from the base station 13(at proper times without delay). In the base station concentrator 12,the subtracter 24 subtracts the count value of the free-running counter22 from the count value reported from the base station 13. Next, thesubtracter 25 subtracts the value outputted from the integrator 27 fromthe count value difference obtained by the subtracter 24. The multiplier26 multiplies together the value obtained by the subtracter 25 and thetime constant α. The frequency deviation between the master clocks ofthe base station concentrator 12 and the base station 13 is calculatedin a longer cycle as the value 1/α is larger (as the value α issmaller). Larger values of 1/α are suitable for applications thatextract stable oscillation frequency differences in longer cycles. Theresults of multiplication by the multiplier 26 are inputted to theintegrator 27, and the input values are continuously added. The resultof addition in the integrator 27 is inputted to the subtracter 25 and tothe control information generator 21.

In the control information generator 21, the average value ofdifferences between the count values of the base station concentrator 12and the base station 13 is converted to a control voltage according tothe table storing the relation between count value difference andcontrol voltage. The information indicating the control voltage isreported to the base station 13 as the synchronization controlinformation.

The synchronization control information sent to the base station 13 isinputted to the corrector 33 of the base station 13. The corrector 33then corrects the control voltage outputted from the DC power supply 32on the basis of the synchronization control information (controlvoltage) sent from the bas station concentrator 12. The correctedcontrol voltage is inputted to the VCO 30, and VCO 30 oscillates at afrequency corresponding to the input control voltage. The clockgenerated by the VCO 30 is the master clock of the base station 13,which is supplied to individual functional parts in the base station 13.

The correction may be applied not to the VCO 30 but to the free-runningcounter 31. In this case, another free-running counter 39 (see FIG. 3),separate from the free-counter 31, is also corrected, and operatingtimings of individual functional parts in the base station 13 areadjusted on the basis of the counter values of the free-running counter39 after corrected.

In the example above, the control information generator 21 is includedin the base station concentrator 12, and the corrector 33 is included inthe base station 13. Assigning the functions in this way is effectivewhen sending the correction voltage instruction to the base station 13requires a very small amount of control data, because the amount of datatransmission between the base station concentrator 12 and the basestation 13 can be optimized. For example, suppose that the VCO 30requires an amount of correction Δv for a frequency deviation Δf. Thecontrol information generator 21 does not give an instruction for thecorrection of Δv at one time, but it gives an instruction for, e.g.,Δv/16, to achieve the correction slowly. This processing enablesoptimization of the amount of data transmission, and also makes itpossible to deal with short-time variations of the output of theintegrator 27, like loss of the output of the free-running counter 39(see FIG. 3).

The control information generator 21 may be included in the base station13, and the corrector 33 may be included in the bas station concentrator12. Including the control information generator 21 in the base station13 is beneficial because differences among individuals units due to VCOcharacteristics etc. can be absorbed within the base stations 13. Whenthe corrector 33 is included in the base station concentrator 12, thebase station concentrator 12 can apply centralized control to thefree-running counters 31 in a plurality of base stations 13, whichfacilitates provision of additional intelligent functions. For example,when the base station concentrator 12 is applying the same control tobase stations 13 with the same VCO configuration, it is possible tocompare operations with other base stations 13 to easily detect abnormalbase stations 13.

FIG. 3 is a diagram illustrating a detailed configuration of the portiondownstream of the VCO in a base station. This diagram shows aconfiguration of a W-CDMA base station 13. The master clock generated bythe VCO 30 in the base station 13 is supplied to individual functionalparts in the base station 13. The base station 13 includes the VCO 30,and a wired interface (I/F) 34, PLLs (Phase-Locked Loops) 35, 36, achannel coding/decoding section 37, a modulator-demodulator 38, and thefree-running counter 39.

The wired interface 34 performs data transmission and reception to andfrom the base station concentrator 12 and the base station controller11. The PLL 35 is a circuit that varies the oscillation frequency of 16MHz outputted from the VCO 30 to 32 MHz. The PLL 36 is a circuit thatvaries the oscillation frequency of 32 MHz outputted from the PLL 35 to64 MHz. The channel coding/decoding section 37 is a W-CDMA channelcoding/decoding component defined by 3GPP. The modulator-demodulator 38is provided separately form the free-running counter 31 used to correctthe output of the VCO 30, and it is a counter that supplies referencetimings to individual functional parts in the base station 13.

Next, the operation illustrated in FIG. 3 will be described.

Supposed that the oscillation frequency of the VCO is originallyspecified to be 16 MHz, and the base station 13 has corrected theoscillation frequency of the VCO 30 by +0.5 ppm, for example. Then, theoscillation frequency 16 MHz+0.5 ppm of the master clock outputted fromthe VCO 30 is inputted to the wired interface 34 that operates with aclock of 16 MHz. The wired interface 34 thus operates at 16 MHz+0.5 ppmthat is timing-adjusted (synchronized) to the master clocks of otherbase stations 13 connected to the base station concentrator 12.

Also, since the oscillation frequency of the output of the VCO 30 is 16MHz+0.5 ppm, the channel coding/decoding section 37 that operates at 32MHz, i.e. twice 16 MHz, operates with the clock of 32 MHz+0.5 ppmoutputted from the PLL 35. It has thus been timing-adjusted to theclocks used by the channel coding/decoding sections 37 of other basestations 13 connected to the base station concentrator 12.

Also, the clock of 32 MHz outputted from the PLL 35 is further varied to64 MHz by the PLL 36, and supplied to the modulator-demodulator 38 thatoperates at 64 MHz. The clock of 64 MHz outputted from the PLL 36 isalso 64 MHz+0.5 ppm because of the correction of the master clock of theVOC 30. Accordingly, the clock supplied to the modulator-demodulator 38is 64 MHz+0.5 ppm, which has also been timing-adjusted to the clocksused by the modulator-demodulators 38 in other base stations 13connected to the base station concentrator 12.

Also, the phase of the free-running counter 39 is adjusted to 16 MHz+0.5ppm from the VCO 30, and so the reference signals to the functionalparts 34, 37, 38 have also been timing-adjusted to reference signals inother base stations 13 connected to the bas station concentrator 12.

Thus, the clocks supplied to all functional parts in the base station 13have been timing-adjusted by correcting the oscillation frequency of theVCO 30, which is more efficient than individually timing-adjustingrespective functional parts by sending a frame synchronization signal.Also, this enhances the performance of radio transmission and receptionbetween the base stations 13 and mobile communications terminals.

In this way, according to the first preferred embodiment, it is possibleto prevent increases of master clock frequency deviations over a longtime, between the base stations 13 and the base station concentrator 12.It is thus possible to match all master clocks of the base stations 13to the timing of the master clock of the base station concentrator 12within a permissible range. It is also thus possible to march theoscillation frequencies of the master clocks among the base stations 13(#1 to #n) within a permissible range. This avoids breakage ofcommunication between the base station controller 11 and the basestations 13 and between the base stations 13 and mobile communicationsterminals.

In particular, it is extremely beneficial because precisesynchronization can be established among the base stations even when thebase stations 13 and the base station concentrator 12 are connectedthrough an asynchronous network such as Ethernet (registered trademark)and clocks cannot be extracted via the circuit.

Also, providing the control information generator 21 and the phasecomparator 23 not in the base stations 13 but in the base stationconcentrator 12 allows reduced circuit scale and lower price of the basestations 13.

Also, it is possible to prevent the base station controller 11 frombecoming unable to recognize the plurality of base stations 13 as asingle base station through the single base station concentrator 12.

Also, it is possible to match the phases even when there is a phaseshift over one period (over 360 degrees) by using high-resolutioncounters in the comparison of phase differences of the master clocks andadjusting till the counter values become the same.

Furthermore, in the W-CDMA, there are following three effects.

Effect (1): Stabilization of wired data send/receive windows.

Effect (2): Stabilization of search windows.

Effect (3): Perfect synchronization at chip level.

Effect (1) means stabilization of the timing width used for datasend/receive between the base station controller 11 and the basestations 13. Effect (2) means stabilization of the window width for pathsearch in the modulator-demodulator 38 in the base stations 13. Effect() means timing synchronization of scrambling code and channelizationcode used in the modulator-demodulators 38 in the base stations 13. TheEffects (1) to (3) will be described in detail below.

FIG. 4 is a diagram illustrating master clocks of base stations. Thediagram shows counter values generated on the basis of the master clocksof two base stations #1 and #2, among a plurality of base stationsconnected to the same base station concentrator 12. Before time T1 atwhich a correction of the master clocks of the base stations 13 starts,there is a gap (Phase difference) between the counter value of the basestation #1 and the counter value of the base station #2. The masterclock correction is accomplished between the time T1 at which thecorrection started and time T2 at which the correction ends. During thisperiod, the oscillation frequency of the VCO 30 of the base station #2is varied. The frequency is increased when the clock phase of the basestation #2 is lagging behind the clock phase of the base station #1, andthe frequency is reduced when the clock phase of the base station #2 isleading ahead of that of the base station #1.

FIG. 5 is a diagram used to explain Effect (1). FIG. 5 shows, from thetop, the counter value of the base station concentrator 12, the countervalue of the base station #1, and the counter value of the base station#2 that are arranged on the time base. FIG 5(a) shows a condition beforethe correction of master clocks of the base stations, and FIG. 5( b)shows a condition after the correction.

In FIG. 5( a), before the time T1 at which the correction of masterclocks of the base stations #1 and #2 starts, the counter value of thebase station #1 and the counter value of the base station #2 differ intiming. Accordingly, when downlink data D is sent from the base stationcontroller 11 that is assuming there is a single base station, the basestation #1 can receive the data D because the receiving timing is withinthe receiving window, but the other base station #2 cannot receive thedata D because the receiving timing is not within the receiving window,and then the communication is broken.

In FIG. 5( b), after the time T2 as which the correction of the masterclocks of the base stations #1and #2 finished, the counter value of thebase station #1 and the counter value of the base station #2 areadjusted to each other such that their receiving windows do notconsiderably differ in timing, and they are in fixed relative positions.Accordingly, when the downlink data D is sent from the base stationcontroller 11 assuming there is a single base station, the receivingtiming of both base stations #1 and #2 are within the receiving windows.The base station controller 11 is thus able to send the downlink data Dto both base stations #1 and #2, and the communication is not broken.

In the conventional technique in which the base station concentratorjust supplies a frame synchronization signal to the lower-level basestation, the timing of the frame synchronization signal varies due tothe circuit between the base station concentrator and the lower-levelbase stations, and so it is impossible to match the receiving windowsamong a plurality of base stations and to fix the relative positions ofthe receiving windows among the plurality of base stations.

For the sake of simplicity, FIG. 4 shows an example in which the periodof “master clock being corrected” corresponds to about three pulses,but, in practice, the correction is achieved slowly (in 3GPP, thecorrection is achieved with counter value=5 per 10⁸ pulses) within arange corresponding to the frequency stability permissible for thecommunications system (in 3GPP, the frequency stability is ±0.05 ppm).

FIG. 6 is a diagram used to explain Effect (2). A base station #1 and abase station #2 are connected to the same W-CDMA base stationconcentrator 12 and the two base stations #1 and #2 are communicatingwith the same mobile communications terminal in a soft handovercondition, and FIG. 6 shows search windows indicating time ranges inwhich the base stations #1 and #2 can demodulate an uplink signal fromthe mobile communications terminal and perform a search for detectingcorrelation pulses of path. The positions of the search windows in timeare determined on the bases of the counter values of the base stations#1 and #2.

FIG. (6)a shows the search window positions of the base stations and thedetected correlation pulses of the path at the time when thecommunication with the mobile communications terminal startedimmediately after the base stations were turned on. In this case, themaster clocks of the base stations have not been corrected, but thecounter values have no deviation yet that would be caused by phase shiftof the master clocks. That is, the search windows of the individual basestations indicate the same timing (assuming that the distances betweenthe individual base stations and the mobile communications terminal areequal), and the individual base stations are both detecting a pathcorrelation pulse. In this condition, the soft handover processsuccessfully functions in which the base stations concentrator 12 makesthe base station controller 11 recognize the base stations as a singlestation.

However, the counter values deviate from each other as time passes,because of a difference in master clock oscillation frequency betweenthe individual base stations. Suppose that, before the counter valuesthus deviate, the base station controller 11 indicated channel settingfor soft handover to the base stations 13 (#1, #22) through the basestation concentrator 12. Offset information (a difference from areference signal of measured by the mobile communications terminalduring the communication with the base station #1 is reported to thebase station controller 11. The base station controller 11 indicates theoffset information of the mobile communications terminal to the basestation #2.

Next, suppose that the communication is continued for a long time, withthe connections being simultaneously made with the base station #1 andbase station #2. When the base station #1 seems to be primary for themobile communications terminal (that is, when the base station #1 seemsto be a primary-cell according to 3GPP), the counter value of the basestation #2 is deviated because of a difference in the oscillationfrequency of the master clock from that of the base station #1.Accordingly, in reality, as shown in FIG. 6( b), its search window isformed in a position shifted from that of the base station #1, and itcannot detect the correlation pulse of the path. Then, the mobilecommunications terminal cannot perform desired communication, though itoriginally ensures its communication quality by soft handover.

When the oscillation frequencies of the master clocks of the basestations are corrected, the positional relation of the search windows ofthe base stations #1 and #2 does not change relatively to each other,and, as shown in FIG. 6( c), it is possible to continuously detect thecorrelation pulse of the path over a long period of time. Thecommunication is not broken even when the mobile communications terminalmoves into a non-overlapping area of one of the base stations. That is,the soft handover functions without any problems. The stabilization ofsearch windows means this effect.

In this conventional technique in which the base station concentratoronly supplies a frame synchronization signal to lower-level basestations, the timing of the frame synchronization signal varies due tothe circuit between the base station concentrator and the lower-levelbase stations, and it is impossible to match the positions of searchwindows among the base stations and to fix the relative positions of thesearch windows.

Now, a situation under the conditions below will be described in detail.

Condition (a): There is a base station having a cell with a small radiusin which the time width of path search by correlating (the width ofsearch window) is 32 chips. The time for 1 chip corresponds to 0.26 μs,and so 32 chips correspond to 8.3 μs.

Condition (b): The phase of the counter value, based on the masterclock, of the base station #1 lags by 3 μs per 10 minutes behind that ofthe base station #2.

Condition (c): The mobile communications terminal is located in theoverlap of the cell of the base station #1 and the cell of the basestation #2, and standing still in a handover position. The base station#1 is the primary-cell.

Condition (d): The base stations #1, #2 and the base stationconcentrator 12 are connected through and IP network. As defined asITU-T recommendation Y.1541 class 0 there is a model in which passagethrough an IP network may cause a delay of 50 ms, and so a drift of 50ms may occur.

Condition (e): The free-running counters 22 and 31 of the base stationsand the base station concentrator are counted up according to masterclocks of 16 MHz. That is, they are counted up by one per 0.0625 μs.

Under the conditions (a) to (e) the master clocks have no phasedifference at the first instance of path detection, and so the searchwindows are in the same positions. Accordingly, the base station #2 isable to detect the path when the chip offset information as pathposition information (assumed to be 10 chips=2.6 μs) is sent from thebase station #1 to the base station controller 11 and the sameinformation is sent to the base station #2.

When there is no function to correct the master clock oscillationfrequencies of the base stations, the free-running counter of the basestation #2 by 2.6 μs after 520 seconds passed, and so it search windowis formed in a position lagging by 2.6 μs behind the search window ofthe base station #1 when the base station #2 receives the chip offsetinformation of 10 chips from the base station controller 11. Then, thebase station #2 cannot detect the path, and the communication betweenthe base station #2 and the mobile communications terminal is broken,and handover fails.

When there is a function to correct the master clock oscillationfrequencies of the base stations, the base station concentrator 12obtains and averages information about the master clock oscillationfrequencies from the base stations for 50,000 ms (=50 seconds) which issufficiently longer than 50 ms, so as to cancel drifts of about 50 ms inthe IP network transmission. When averages, the difference between thecounter values of the base station #1 and the base station concentrator12 was +6 (0.375 μs), and the difference between the counter values ofthe base station #2 and the base station concentrator 12 was ±2 (0.125μs). Then, on the basis of the table storing a relation between countervalue difference and VCO control voltage, the base station concentrator12 applies a correction of −0.006 V (an example) to the base station #1to shorten the master clock cycle by 0.375 μs÷50 seconds=0.0075 μs justonce, and also applies a correction of −0.002 V (an example) to the basestation #2 to shorten the master clock cycle by 0.125 μs÷50seconds=0.0025 μs. In this way, the master clock oscillation frequenciesof the base station #1 and the base station #2 are corrected once per 50seconds, and the search window of the base station #2 is corrected oncein every 50 seconds such that it will not deviate in position from thesearch window of the base station #1.

Now, a further effect applied to both Effects (1 ) and (2) will bedescribed.

On the occurrence of IP routing table variations, variations of averagedelay time due to IP network load variations (due to huge packets likeFTP (File transfer protocol)), drifts between the base stationconcentrator 12 and base stations 13, and the like, the averagingoperation is performed once for every 50 seconds as explained above,wherein, for example, corrections are applied according to thedifferences between counter values by the averaging in the first 50seconds (for example, a correction of control voltage of −0.006 V for acounter value difference=+6 in the base station #1, and a correction ofcontrol voltage of −0.002 V for a counter value difference=+2 in thebase station #2), and then, an average delay time occurs, andcorrections are applied according to the differences by the averaging inthe next 50 seconds (for example, a correction of control voltage of−0.002 V for a counter value difference=+2 in the base station #1, and acorrection of control voltage of 0.0 V for a counter value difference=±0in the base station #2).

The averaging also suppresses bursting data delay drifts.

Also, it is necessary to remove fixed delays to some extent, in order toestablish synchronization among base station. Otherwise, it is notpossible to deal with condition in which, for example, the base station#1 has a fixed delay of 100 μs and the base station #2 has a fixed delayof 30 ms. Such conditions can be dealt with by providing the basestation concentrator 12 with a time information server function, such asthe NTP (Network Time Protocol) server function. When a lower-level basestation 13(#1) connected to the base station concentrator 12 sends arequest for time to the base station concentrator 12, the base station13 is informed of time. Then the fixed delay of the base station 13 canbe removed to some extent, and the count value differences between thebase station 13 and the base station concentrator 12 can be preciselycalculated. The quantity of such fixed delay is set as counter initialvalue when the base station 13 is activated.

FIG. 7 is a diagram illustrating a fixed delay between a base stationand the base station concentrator. FIG. 8 is a diagram used to explain aprocedure for removing the fixed delay. In FIG. 8, the base stationconcentrator 12 shown in FIG. 2 further includes a time adjustmentsection 40 having a time information server function. Now, the procedurefor removing fixed delay will be sequentially described referring toFIGS. 7 and 8.

Step (1): The base station concentrator 12 sends an inquiry signal toask a lower-level base station 13 for a request for time information.

Step (2): The base station 13 sends a time information request to thebase station concentrator 12 (send time A1). The time informationrequest is received at the base station concentrator 12 and transferredto the time adjustment section 40 (receive time M1).

Step (3): The time adjustment section 40 of the base stationconcentrator 12 generates time information according to the request, andsend the generated time information back to the base station 13 as aresponse to the request (send time M2). The time information response isreceived at the base station 13 (receive time A2).

Step (4): The base station 13 corrects the time on the basis of the timeinformation thus sent. In this process, the amount of correction of thetime of the base station 13 is calculated according to the expressionbelow:((A2−A1)−(M2−M1))/2

It should be noted that the propagation delays of data transmission andreception of the base station 13 and the base station concentrator 12are assumed to be equal. Accordingly, it is not always possible to makethe time of the base station 13 precise, but it is possible to removethe fixed delay in data transmission between the base station 13 and thebase station concentrator 12 by subtracting the calculated fixed delaytime form the counter value of the base station 13.

It is also effective to calculate IP network fixed delay time byproviding the base stations 13 with a time information server function.

It is also effective to cause the base station concentrator 12 torequest time information from the base station 13, receive timeinformation from the base station 13, calculate the fixed delay, andsend the result to the free-running counter 31 of the base station 13,so as to correct the counter value of the free-running counter 31 of thebase station 13.

It is effective to activate the fixed delay calculating process at sometime intervals. It is also effective to obtain an average a sufficientnumber of time and over a sufficient time period, so as to reduce theinfluences of loss of IP packets or the influences of bursting drifts offixed delay calculation packets.

Next, Effect (3) will be described referring to FIGS. 9 to 11.

For Effect (3), “chip” means one pulse after spread in CDMA.Hereinafter, a state in which synchronization is established at the choplevel is referred to as chip-level synchronization.

When perfect chip-level synchronization is established such complicatedsituations as shown in FIG. 9 will not occur where the base station #1sends Timing-adjustment for downlink data, while the base station #2does not send Timing-adjustment for the downlink data, because of aslight deviation of windows for wired data transmission/reception.

When chip-level synchronization is perfectly established and the basestations 13 (#1, #2) have small-radius cells as shown in FIG. 10, thechip-offset information and frame-offset information detected by themobile communications terminal 14 can be used in the base station towhich it has been handed over, which shortens the search time.

Also, when perfect chip-level synchronization is not established, asshown in FIG. 11, considering the maximum ratio combining performed witha signal sent to the base station concentrator 12 from a base station #1located relatively distant and having a larger propagation delay and asignal sent to the base station concentrator 12 from a base station #2located relatively near an having a smaller propagation delay, it isnecessary to provide FIFO (First In First Out) buffers 42 (#1, #2) forqueuing, in respective correspondence with the base stations #1 and #2in a stage preceding the maximum ratio combining section 41.

In contrast, when chip-level synchronization is perfectly established,the delays between the base station concentrator 12 and the basestations 13 are small, and it is not necessary to provide such queuingFIFO buffers 42 as shown in FIG. 11 for the maximum ratio combiningprocess performed by the maximum ratio combining section 41 in the basestation concentrator 12, with soft decision information sent to the basestation concentrator 12.

According to the present invention, in correcting the oscillationfrequencies of the master clocks of base stations 13, an averaging isperformed for considerably longer periods as compared to network drifts,in order to remove data loss and drifts caused in the IP network.Accordingly, Effect (3) is effective when it is known in advance that,as a system, data loss and drifts in the IP network between the basestation concentrator 12 and the base stations 13 are suppressedsufficiently lower than chip level (when the communication does not passvia a hub or the like, or when the hub has an extremely high processingability and ensures low delay, for example).

Second Preferred Embodiment

FIG. 12 is a diagram illustrating the configuration of a base stationsynchronization system according to a second preferred embodiment of thepresent invention. In the second preferred embodiment, a base stationconcentrating function section 52, which corresponds to the base stationconcentrator 12 of the first preferred embodiment (see FIG. 1), isincorporated in a base station controller 11. The base stationcontroller 11 of the first preferred embodiment corresponds to a basestation controlling function section 51 in the second preferredembodiment. The base station controlling function section 52 of the basestation controller 11 and base stations 13 are W-CDMA equipment definedby 3GPP, for example. The base stations 13 and the base stationcontroller 11 are connected through an IP network, for example. In otherrespects, the configurations, operations and effects are the same asthose of the first preferred embodiments and so not described hereagain.

In this way, the base station concentrator 12 can be omitted when it isnot necessary to connect a large number of base stations 13 to the basestation controller 11.

The VCO 20 (which can be an OCXO or the like when voltage control is notrequired) and the control information generator 21 shown in FIG. 12, andthe free-running counter 22 and part of the phase comparator 23, may beprovided not within the base station concentrating function section 52,but may be provided outside the base station concentrating functionsection 52 but within the base station controller 11.

Third Preferred Embodiment

FIG. 13 is a diagram illustrating the configuration of a base stationsynchronization system according to a third preferred embodiment of thepresent invention. In a third preferred embodiment, the VCOs 20, 30 ofthe first preferred embodiment (see FIG. 1) are replaced byhigh-precision VCOs 60. The VCOs 60 are, for example, VC-OCXO(voltage-controlled, oven-controlled crystal oscillators) or VC-DTCXO(voltage-controlled, digital temperature-compensated crystaloscillators), which are capable of generating high-precision masterclocks. In other respects, the configurations, operations and effectsare the same as those of the first preferred embodiment and are notdescribed again here. In this preferred embodiment, too, the oscillatorin the bas station concentrator 12 may be an OCXO, DTCXO or the like, inplace of VCO, when voltage control is not required.

The free-running counter frequency is used for RF (Radio Frequency)transmission reference, and do the frequency is changed at a rate atwhich frequency deviations can be kept permissible. This changing rateis very low, and when the frequency is change by 0.01 ppm, for example,the correction can be made only by 36 μs per hour. Accordingly, withhigh-precision clocks, it is not necessary to very frequently correctthe master clock oscillation frequencies in the base stations 13.

The high-precision VCO 60 may be applied only to the base stations 13,or only to the base station concentrator 12. This offers enhancesprecision in the correction of master clock oscillation frequencies,than when the high-precision VCO 60 is not applied.

Fourth Preferred Embodiment

FIG. 14 is a diagram illustrating the configuration of a base stationsynchronization system according to a fourth preferred embodiment of thepresent invention. In the fourth preferred embodiment, a selector 71 isadded to the first preferred embodiment (see FIG. 2), and the phasecomparator 23 is replaced by a phase comparator 72.

Among counter values of the free-running counter 31 of the base station13, the selector 71 sends a counter value inputted at a certain time asan initial value to a free-running counter 22 of the base stationconcentration 12. Also, after a certain time has passed after sendingthe initial value to the free-running counter 22, the selector 71 againsends the count value of the free-running counter 31 of the base station13 to the phase comparator 72 in the base station concentrator 12. Theselector 71 may be integrally incorporated in the phase comparator 72.

Like the phase comparator 23 of the first preferred embodiment, thephase comparator 72 compares phase information from the free-runningcounter 22 of the base station concentrator 12 with phase informationfrom the free-running counter 31 of the base station 13. However, thephase comparator 72 is formed only of the subtracter 24, among thecomponents of the phase comparator 23 of the first preferred embodiment.

In other respects, the configurations, operations and effects are thesame as those of the first preferred embodiment, and so not describedhere again.

The selector 71 can be omitted when the base station 13 is configured tosupply the count values to the base station concentrator 12 at certaintime intervals. Alternatively, the base station concentrator 12 mayspecify the timing of supply of count values from the base station 13 tothe base station concentrator 12. In this case, the base station 13supplies its counter values to the base station concentrator 12according to a timing based on the instruction from the base stationconcentrator 12.

While FIG. 14 illustrates a configuration including the base stationconcentrator 12 in addition to the base station controller 11, a basestation concentrating function section 52 may be incorporated in thebase station controller 11 as shown in the second preferred embodiment(see FIG. 12).

Fifth Preferred Embodiment

FIG. 15 is a diagram illustrating the configuration of a base stationsynchronization system according to a fifth preferred embodiment of thepresent invention. In the fifth preferred embodiment, the controlinformation generator 21, the selector 71, and the phase comparator 72,which are provided in the base station concentrator 12 in the fourthpreferred embodiment (see FIG. 14), are provided in the base station 13.

Among counter values sent from the free-running counter 22 of the basestation concentrator 12, the selector 71 sends a counter value inputtedat a certain time as an initial value to the free-running counter 31 ofthe base station 13. Also, after a certain time has passed, the selector71 again sends the count value sent from the free-running counter 22 ofthe base station concentrator 12 to the phase comparator 72 in the basestation 13. The selector 71 may be integrally incorporated in the phasecomparator 72.

The phase comparator 72 compares phase information from the free-runningcounter 31 of the base station 13 with phase information from thefree-running counter 22 of the base station concentrator 12. However,the phase comparator 72 if formed only of the subtracter 24, among thecomponents of the phase comparator 23 of the first preferred embodiment.

In other respects, the configurations, operations and effects are thesame as those of the first preferred embodiment, and so not describedhere again. Characteristically, the fifth preferred embodimentsimplifies the configuration of the base station concentrator 12 andrealizes lower price.

The selector 71 can be omitted when the base station concentrator 12 isconfigured to supply count values to the base station 13 at certain timeintervals. Alternatively, the selector 71 may be configured to switchits output according to an instruction from the base stationconcentrator 12.

BRIEF DESCRIPTION OF THE DRAWINGS

[FIG. 1] A diagram illustrating a network configuration according to afirst preferred embodiment of the present invention.

[FIG. 2] A diagram illustrating detailed configurations of a basestation concentrator and a base station.

[FIG. 3] A diagram illustrating detailed configuration of the VCO andfollowing components of the base station.

[FIG. 4] A diagram illustrating master clocks of base stations.

[FIG. 5] A diagram used to explain Effect (1).

[FIG. 6] A diagram used to explain Effect (2).

[FIG. 7] A diagram illustrating fixed delay between a base station andthe base station concentrator

[FIG. 8] A diagram used to explain a procedure for removing fixed delay.

[FIG. 9] A (first) diagram used to explain Effect (3).

[FIG. 10] A (second) diagram used to explain Effect (3).

[FIG. 11] A (third) diagram used to explain Effect (3).

[FIG. 12] A diagram illustrating the configuration of a base stationsynchronization system according to a second preferred embodiment.

[FIG. 13] A diagram illustrating the configuration of a base stationsynchronization system according to a third preferred embodiment.

[FIG. 14] A diagram illustrating the configuration of a base stationsynchronization system according to a fourth preferred embodiment.

[FIG. 15] A diagram illustrating the configuration of a base stationsynchronization system according to a fifth preferred embodiment.

Description of the Reference Characters

11 base station controller

12 base station concentrator

13 base station

14 mobile communication terminal

20 VCO (Voltage-Controlled Oscillator)

21 control information generator

22 free-running counter

23 phase comparator

24,25 subtracter

26 multiplier

27 integrator

30 VCO (Voltage-Controlled Oscillator; master clock generator)

31 free-running counter

32 DC power supply

33 corrector

34 wired interface (I/F)

35,36 PLLs (Phase-Locked Loops)

37 channel coding/channel decoding section

38 modulator-demodulator

39 free-running counter

40 time adjustment section

41 maximum ratio combining section

42 FIFO buffer

51 base station controlling function section

52 base station concentrating function section

60 VCO (high-precision CLK generation)

71 selector

72 phase comparator

D data

A1 send time (time information request)

M1 receive time (time information request)

M2 send time (time information response

It information

1. A base station synchronization system comprising: a plurality of basestations; a synchronization controller that provides control toestablish synchronization among said plurality of base stations, saidsynchronization controller including: a control information generator togenerate synchronization control information for synchronizing masterclocks of said plurality of base stations with each other, thesynchronization control information using an average difference betweena count value corresponding to a first frequency associated with thesynchronization controller and a count value corresponding to a secondfrequency associated with a base station from the plurality of basestations; and each base station from said plurality of base stationsincluding: a master clock generator that oscillates at a frequencycorresponding to an input control voltage, and a control voltagecorrector that corrects the input control voltage to said master clockgenerator in accordance with the synchronization control informationgenerated by said synchronization controller.
 2. The base stationsynchronization system according to claim 1, wherein saidsynchronization controller is a higher-level device for said basestations.
 3. The base station synchronization system according to claim2, wherein said synchronization controller is a base stationconcentrator that relays between said plurality of base stations and abase station controller.
 4. The base station synchronization systemaccording to claim 2, wherein said synchronization controller is a basestation controller that controls said plurality of base stations.
 5. Thebase station synchronization system according to claim 1, wherein eachbase station from said plurality of base stations further includes: aphase information generator to generate phase information about a masterclock oscillated by said master clock generator, and said controlinformation generator of said synchronization controller generates thesynchronization control information according to the phase informationgenerated by said phase information generator.
 6. The base stationsynchronization system according to claim 5, wherein saidsynchronization controller further includes: a clock generator tooscillate a clock at the first frequency, and a phase comparator tocompare phase information about the clock oscillated by said clockgenerator with the phase information generated by said phase informationgenerator of each base station from said plurality of base stations, andsaid control information generator of said synchronization controllergenerates the synchronization control information according to a resultof phase comparison made by said phase comparator.
 7. The base stationsynchronization system according to claim 6, wherein said phasecomparator outputs an average among a plurality of results of the phasecomparison.
 8. The base station synchronization system according toclaim 7, wherein said phase comparator is capable of setting anaveraging cycle.
 9. The base station synchronization system according toclaim 5, wherein said synchronization controller further includes: atime information providing section to provide time information, and thephase information about the clock oscillated by said master clockgenerator of each base station from said plurality of base stations iscorrected on a basis of the time information provided by said timeinformation providing section of said synchronization controller. 10.The base station synchronization system according to claim 1, whereinsaid synchronization controller further includes: a clock generator tooscillate a clock at the first frequency, and said control informationgenerator of said synchronization controller generates thesynchronization control information according to phase information aboutthe clock oscillated by said clock generator.
 11. The base stationsynchronization system according to claim 10, wherein each base stationfrom said plurality of base stations further includes: a phasecomparator to compare phase information about the clock oscillated bysaid master clock generator with phase information based on saidsynchronization control information, and said control voltage correctorof each base station from said plurality of base stations corrects theinput control voltage to said master clock generator in accordance witha result of the phase comparison made by said phase comparator.
 12. Thebase station synchronization system according to claim 11, wherein saidphase comparator outputs an average among a plurality of results of thephase comparison.
 13. The base station synchronization system accordingto claim 12, wherein said phase comparator is capable of setting anaveraging cycle.
 14. The base station synchronization system accordingto claim 11, wherein said synchronization controller further includes: atime information providing section to provide time information, and thephase information about the clock oscillated by said master clockgenerator of each base station from said plurality of base stations iscorrected on a basis of the time information provided by said timeinformation providing section of said synchronization controller. 15.The base station synchronization system according to claim 1, whereinthe average difference is determined using an adjustable averagingperiod.
 16. A synchronization controller that provides control toestablish synchronization among a plurality of base stations, saidsynchronization controller comprising: a control information generatorto generate synchronization control information for synchronizing masterclocks of said plurality of base stations with each other, thesynchronization control information using an average difference betweena count value corresponding to a first frequency associated with thesynchronization controller and a count value corresponding to a secondfrequency associated with a base station from the plurality of basestations.
 17. The synchronization controller according to claim 16,wherein the average difference is determined using an adjustableaveraging period.
 18. A base station that synchronizes with another basestation, the base station comprising: a master clock generator thatoscillates at a frequency corresponding to an input control voltage; anda control voltage corrector that corrects the input control voltage tosaid master clock generator in accordance with synchronization controlinformation that is provided from a synchronization controller, thesynchronization control information using an average difference betweena count value corresponding to a first frequency associated with thesynchronization controller and a count value corresponding to a secondfrequency associated with the base station.
 19. The base stationaccording to claim 18, wherein the average difference is determinedusing an adjustable averaging period.